A hallmark of early vertebrate development is the progressive growth of the embryonic body from the anterior (A) to the posterior (P), which relies on a multipotent stem-like progenitor population located at the most posterior end of the embryo, in a region called the tailbud. This progenitor population gradually releases mesodermal cells that populate the somites (primarily muscle), as well as neural cells that form the spinal cord, until the complete A-P axis has been established. For the body to form normally, both the rate of release from the tailbud and cell proliferation must be carefully controlled so tht the correct proportion of cells is produced along the entire A-P axis. How these processes are regulated and integrated is poorly understood. The first aim of this proposal will examine the mechanisms by which cells leave the tailbud and enter the somites. Based on recent preliminary results with a new transgenic line, the role of Wnt signaling in regulating this process will be elucidated by testing two hypotheses for Wnt function. In addition, analysis of a small unique element within the tbx16/spadetail promoter, which is activated just as cells make the decision to leave the progenitor population, will provide key insight into the molecular mechanism that regulates the initial step in the commitment to the mesodermal fate as the body elongates. Identifying the mechanisms that control cell allocation will be a major step forward in understanding how the vertebrate embryo precisely regulates the release of cells from the tailbud. The second aim will determine why cell proliferation is tightly regulated in the post-gastrula embryo such that the progenitors are quiescent, and only divide when they first begin to differentiate. This aim will test the hypotheses that this regulation is essential to allow normal signaling and/or morphogenesis of the mesodermal progenitors and their derivatives using a novel transgenic line we have recently produced. Determining why the vertebrate embryo strictly controls proliferation is essential for understanding how the embryo regulates the competing needs to increase cell number yet maintain the complex signaling and morphogenetic processes necessary to establish the embryonic body plan. Studies, particularly in mammals, show that the posterior progenitors are a stem-cell like population, which contribute to a variety of cell types. With the ability to produce transgenic lines expressing temporally controlled regulators of signaling and cell proliferation, as well as the ability to easily knock down gene function, zebrafish provides an excellent model system for understanding how vertebrate stem cells are regulated in vivo. As stem cells have great promise for the treatment of many diseases, the studies described here will provide valuable information about the signaling networks and regulatory factors that control stem cell maintenance and tissue formation.